JP2009074155A - Method for quenching die - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for quenching a die with which even in the case of being a die having complicated shape, uniform crystal grain can be obtained. <P>SOLUTION: In the method for quenching the die, after the quenching temperature raising process, in which the die is heated in the temperature range from A1 transforming point to the A3 transforming point at ≥100°C/H heating rate, a holding process for holding the die in the temperature range of A3 transforming point to <1150°C, is performed and successively, a quenching-cooling process, in which the die is cooled in the temperature range from A3 transforming point to 600°C at 5-20°C/min cooling rate, is performed, and after passing through an interrupting holding process in the temperature range of 500-400°C for 0.5-5 hr, a low temperature quenching-cooling process, in which the die is cooled in the temperature range of 400-200°C at 1-15°C/min cooling rate, is performed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a mold quenching method, and more particularly to a mold quenching method capable of refining crystal grains in addition to low strain and high toughness.

Molds are required to have high hardness and high toughness, and their characteristics are greatly influenced by the heat treatment quenching method.
In the quenching heating, a high quenching temperature is selected within a range in which the crystal grains do not become coarse in order to dissolve the alloy elements to the maximum. In order to obtain high toughness, it is necessary to suppress carbide precipitation at the crystal grain boundary and prevent bainite transformation in quenching and cooling at the same time as crystal grains are refined. At this time, quenching / cooling needs to be rapidly cooled, but on the other hand, if the metal is rapidly cooled, distortion and deformation of the mold increase, and therefore the cooling rate needs to be controlled appropriately. For this reason, various proposals have been conventionally made.
Conventionally, various proposals have been made as methods for obtaining a mold that requires both low strain and high toughness. Most of the conventionally proposed methods are to achieve both low strain and high toughness by adjusting the cooling conditions from the quenching temperature.
For example, recently, in Japanese Patent Application Laid-Open No. 2006-342377 (Patent Document 1), the cooling following the heating to the quenching temperature is performed. The cooling rate of the slowest part of the cooling is 20 to 20 in the high temperature region from the quenching temperature to 600 ° C. By implementing the low temperature region from 5 ° C / min to 400 ° C to 200 ° C at 1 to 15 ° C / min, it is possible to obtain a mold having low strain and high toughness by avoiding cracks. A marquenching method has been proposed.
The marquenching method is a process in which the cooling during quenching cooling is held at the upper part of the martensite transformation or isothermally at a temperature slightly higher than that in order to prevent quench cracking due to rapid cooling, and then cooled after the temperature of each part has become uniform. It is.
JP 2006-342377 A

The method disclosed in Patent Document 1 described above is also a general marquenching method, which is a low-distortion mold quenching method obtained by the marquenching method.
The present inventor has also studied a mold quenching method that achieves both low strain and high toughness by utilizing the marquenching method. Certainly, by applying the marquenching method, both low strain and high toughness can be achieved. I found out. However, it has been found that in a mold having a complicated shape, even if only the cooling rate is adjusted, for example, there is a variation in crystal grains near the center and the center of the mold, and accordingly, there is also a variation in toughness. did.
An object of the present invention is to provide a method for quenching a mold that can be adjusted to uniform crystal grains even if the mold has a complicated shape.

The present inventor examined various heat treatment conditions as a technique for suppressing the variation while obtaining excellent toughness in order to make the metal structure of a mold having a complicated shape uniform. As a result, it was found that not only the cooling conditions but also the heating conditions are important in quenching the mold, and the present invention has been achieved.
That is, according to the present invention, in the mold quenching method, after the quenching and heating step in which the temperature range from the A1 transformation point to the A3 transformation point is heated at a heating rate of 100 ° C./H or higher, the temperature exceeds the A3 transformation point and exceeds 1150 ° C. A holding process for holding in a non-temperature range is performed, and then a quenching cooling process is performed in a temperature range from the A3 transformation point to 600 ° C. at a cooling rate of 5 to 20 ° C./min, and in a temperature range from 500 to 400 ° C. This is a mold quenching method through a low temperature side quenching cooling process in which a temperature range of 400 to 200 ° C. is cooled at a cooling rate of 1 to 15 ° C./min after an interruption and holding step of 0.5 to 5 hours.
In the present invention, it is preferable that the quenching temperature raising step of heating the temperature range from the A1 transformation point to the A3 transformation point is heated at a heating rate of 150 ° C./H or more, thereby improving productivity. Also, the crystal grains become finer and stable quality can be obtained.
In the present invention, preferably, the heating in the low temperature side quenching temperature raising step performed before the quenching temperature raising step for heating the temperature range from the A1 transformation point to the A3 transformation point is performed at a heating rate of 150 ° C./H or less. This is a quenching method for molds.
More preferably, it is a mold quenching method in which a quenching temperature rise preheating step of holding at least once is performed before the quenching temperature raising step of heating the temperature range from the A1 transformation point to the A3 transformation point.

  According to the mold quenching method of the present invention, since the crystal grains remain fine even when the quenching temperature is increased, high toughness is obtained and large cracks are prevented when a large load is applied to the mold. it can. In addition, since the quenching temperature can be increased, the hardness and the high temperature hardness are also high, which is effective in suppressing heat cracks and the like. In addition, the reduction in heat treatment strain is effective in reducing the number of repairs after heat treatment.

As described above, an important feature of the present invention is that the speed at which the temperature is raised to the quenching temperature is optimized in the mold quenching method. The present invention will be described below.
In the present invention, the reason for heating the quenching temperature raising step (FIGS. 1 (2) and 2 (2)) from the A1 transformation point to the A3 transformation point at a heating rate of 100 ° C./H or more is as follows. Is due to.
In the present invention, the condition of heating to the quenching temperature is particularly important. In order to refine crystal grains and suppress variation in crystal grain size, it is necessary to control the generation and growth of austenite generated by transformation in the temperature range from the A1 transformation point to the A3 transformation point.
As a condition for this, it is necessary to heat the temperature range from the A1 transformation point to the A3 transformation point at a heating rate of 100 ° C./H or more. This is because when new austenite crystal grains are produced from ferrite, if the heating rate is high, the nucleation density of austenite is high due to the overheating effect from the equilibrium temperature, and the effect of refining the crystal grains is obtained.

When the heating rate in the quenching temperature raising step of the present invention is less than 100 ° C./H, or when preheating is held between the A1 transformation point and the A3 transformation point, there are few nuclei generating austenite, one by one. One crystal grain grows large, and after the austenite transformation is completed, the crystal grain becomes coarse and the crystal grain size tends to vary. Therefore, in the present invention, the temperature range from the A1 transformation point to the A3 transformation point is heated at a heating rate of 100 ° C./H or more. Although it depends on the shape and weight of the material to be heat-treated, the temperature can be increased at a heating rate of 150 ° C./H or more, productivity can be improved, and crystal grains are finer, which is further preferable.
The upper limit of the heating rate varies depending on the performance of the heating furnace and the weight and shape of the material to be heat treated, so it cannot be determined unconditionally, but if the heating rate is excessively increased, distortion due to heating unevenness of the material to be heat treated will occur. Since it becomes easy to generate | occur | produce, it is good to make 400 degreeC into the upper limit of a heating rate from experience, Preferably it is 300 degreeC / H, More preferably, it is 280 degreeC / H, More preferably, it is 250 degreeC / H.

As described above, in the present invention, the conditions for heating to the quenching temperature are important.
As a condition for heating to a more preferable quenching temperature of the present invention, the temperature rising condition of the low temperature side quenching temperature rising process (FIG. 1 (1), FIG. 2 (1)) performed before the above-described quenching temperature rising process is also adjusted. Further preferred.
The condition of the low temperature side quenching temperature raising step is preferably a temperature rising rate of 150 ° C./H or less. This is because the heat-treated material is distorted, or the temperature difference between the surface layer portion and the inside of the heat-treated material is increased, which increases the possibility of crystal grain variation depending on the site.
A preferable temperature increase rate in the low temperature side quenching temperature increasing step is in the range of 50 to 150 ° C./H. More preferably, it is in the range of 75 ° C. ± 25 ° C./H, and more preferably in the range of 75 ° C. ± 15 ° C./H.

Moreover, in this invention, you may perform the quenching temperature rising preheating process which hold | maintains at least 1 time in the low temperature side temperature range of the above-mentioned quenching temperature rising process (FIG.1 (2), FIG.2 (2)). (FIG. 2 (1A)).
By performing the quenching temperature increase preheating step (FIG. 2 (1A)), the temperature unevenness when the heat-treated material is heated is reduced, so that deformation is reduced. Further, by preheating the processing residual stress generated at the time of manufacturing the mold, the strain is removed, and there is also an effect of suppressing abnormal growth of crystal grains using the residual strain as a driving force when passing through the transformation point by subsequent heating.
In order to obtain this effect more reliably, the temperature rise preheating step is preferably performed in the temperature range of A1 transformation point −15 ° C. to −200 ° C. More preferably, it is the temperature range of A1 transformation point -20 degreeC--70 degreeC.
In addition, since the holding time of the quenching temperature rising preheating step is intended to reduce the temperature unevenness when the heat-treated material is heated as described above, it is difficult to obtain the effect of reducing the temperature unevenness if the time is too short. Become. Therefore, it is preferable that the time is sufficient to reduce temperature unevenness. However, since it varies depending on the weight and shape of the material to be heat-treated, it cannot be determined unconditionally, but from experience, it is preferable to hold for about 0.5 to 5 hours. If held for 0.75 hours or longer, the difference between the surface layer temperature and the internal temperature can be kept within 30 ° C., so it is preferable to hold it for 0.75 hours (45 minutes) or longer.
For example, when the method of the present invention is actually applied, for example, the material to be hardened is placed in a heating furnace preheated to about 400 to 500 ° C., and is kept isothermally at the temperature of the preheating furnace. It is preferable from the viewpoint of productivity to select a method for reducing the temperature difference from the inside. Even when this low temperature preheating is performed, it is preferable to apply the quenching temperature rising preheating step in the temperature range of −15 ° C. to −200 ° C. of the A1 transformation point.

Next, the holding process at the quenching temperature (FIGS. 1 (3) and 2 (3)) will be described.
The temperature of the holding step at the quenching temperature of the present invention is set to a temperature range not lower than 1150 ° C. above the A3 transformation point.
This is because if the quenching temperature holding temperature is lower than the A3 transformation point temperature, the solid solution of carbide and alloy elements is insufficient, the hardness is low, and the high temperature strength is also low, so heat cracks are likely to occur. When the quenching heating temperature exceeds 1150 ° C., the carbide pinning the crystal grains also dissolves and the crystal grains grow abnormally.
In order to suppress the occurrence of these problems and achieve the refinement of crystal grains, a temperature range of A3 transformation point to 1150 ° C. is required. Preferably it is 1010-1050 degreeC.

Next, the quenching and cooling process of the present invention will be described.
In the present invention, after the holding step at the quenching temperature, as the quenching cooling step, the temperature range from the A3 transformation point to 600 ° C. is cooled at a cooling rate of 5 to 20 ° C./min. (Fig. 1 (4), Fig. 2 (4))
In this temperature range, when the cooling rate is slow, carbides are precipitated at the crystal grain boundaries, the grain boundary fracture is likely to occur, and the toughness and the stress corrosion cracking resistance are lowered. In order to prevent this, a cooling rate of at least 5 ° C./min is required. When the temperature exceeds 20 ° C./min, the variation in cooling increases, the temperature unevenness between the surface layer portion and the inside of the heat-treated material increases, and the distortion and shape increase due to the difference in thermal stress during cooling.
Accordingly, the A3 transformation point is a cooling condition that can be maintained as it is in the process up to the holding process, maintaining the effect of homogenizing the crystal grains, relaxing the distortion of the thermal stress difference during cooling, and preventing deformation. To 600 ° C. was defined as cooling at a cooling rate of 5 to 20 ° C./min. The cooling condition is preferably 10 to 15 ° C./min.

Subsequently, in the present invention, an interruption holding process is performed. (Fig. 1 (5), Fig. 2 (5))
The interruption holding process aims to eliminate temperature unevenness and reduce the temperature difference between inside and outside by holding the thermal stress generated in the quenching cooling process substantially isothermally.
The reason why the temperature of the interruption holding process is set to 500 to 400 ° C. is that this temperature region is a region in which austenite is metastable. When held at a temperature higher than 500 ° C., pearlite transformation may occur, whereas when it is lower than 400 ° C., bainite transformation starts. Therefore, the temperature of the interruption holding process of the present invention is limited to the range of 500 to 400 ° C. Preferably it is a temperature range of 425-475 degreeC.
The time for this interruption holding step is at least 0.5 hours in order to obtain the effect of uniformizing the temperature of the heat-treated material. The longer the time is, the better the temperature is, but even if it is kept for 5 hours or more, the effect of temperature uniformity is saturated, so the upper limit is set to 5 hours in consideration of practical productivity. . If held for 0.75 hours or longer, the difference between the surface layer temperature and the internal temperature can be kept within 30 ° C., so it is preferable to hold it for 0.75 hours (45 minutes) or longer.

Next, a low-temperature side quenching cooling step is performed in which the temperature range of 400 to 200 ° C. is cooled from the holding temperature in the interruption holding step at a cooling rate of 1 to 15 ° C./min. (Fig. 1 (6), Fig. 2 (6))
The cooling rate of this low-temperature side quenching cooling process suppresses the generation of bainite of the material to be heat-treated during cooling, suppresses temperature unevenness due to rapid cooling, and is necessary to control toughness, quenching distortion, and cracking. Speed.
At a cooling rate of less than 1 ° C./min, the formation of bainite is large and the toughness is reduced. If it exceeds 15 ° C./min, the temperature difference of the product during the martensitic transformation increases, and due to temperature unevenness during cooling, distortion tends to increase and the risk of burning cracks also increases.
A preferable cooling rate is 10 to 15 ° C./min.

As described above, when the quenching method of the present invention described above is applied, even a mold having a complicated shape can be adjusted to uniform crystal grains.
For example, in order to adjust to the conditions of the quenching and cooling step described above, after the uniform cooling while rotating with a fan until the maximum surface temperature of the material to be heat-treated becomes 600 ° C. or less, the heating previously maintained at 500 to 400 ° C. Hold in the furnace, temporarily stop cooling and hold it for an interrupted holding process, hold for a predetermined time, then restart cooling, quench into quenching oil, or pressurize other than fan or quenching oil It is good also as a low-temperature side quenching cooling process by forcedly convection with an inert gas and cooling.

The following examples further illustrate the present invention.
First, blocks each having a thickness of 300 mm (w) x 300 mm (l) x 300 mm (t) are cut out from the die material for hot dies having the composition shown in Table 1, and the width is parallel to the forging direction on one surface thereof. Was prepared by simulating a mold by machining a groove having a depth of 50 mm and a depth of 50 mm.
No. Alloy A is a JIS SKD61 equivalent material. Alloy 2 is an alloy with improved heat crack resistance, in which Co and Ni are added to SKD61 and Mo is further increased.
No. The A1 transformation point of the alloy A is 850 ° C. and the A3 transformation point is 895 ° C. The B1 alloy had an A1 transformation point of 830 ° C and an A3 transformation point of 850 ° C.

These block samples were individually quenched using a vacuum furnace.
In order to measure the cooling rate, it was measured by inserting a thermocouple at the center. The surface temperature was confirmed with a radiation thermometer. The quenching conditions are shown in Table 2. In addition, the low temperature side quenching temperature rising process (FIG. 1 (1), FIG. 2 (1)) conditions which are not shown in Table 2 were 75 degreeC / H.
No. in Table 2 The test piece 6 is heated to the quenching temperature in a vacuum or in an inert gas atmosphere, and the gas pressurization amount is controlled in the inert gas atmosphere in the subsequent quenching cooling process and the low temperature side quenching cooling process after the interruption holding process. While cooling.
The other cooling conditions for the test specimens are as follows: As a quenching cooling process, the material is once taken out of the heating furnace, air-cooled or blast-cooled in the atmosphere, suspended in the middle of cooling, and installed in a heating furnace set at a predetermined temperature. After passing through an interrupting holding process for heating and holding, it was cooled with quenching oil to form a low-temperature quenching cooling process.
The heat pattern of each test piece is No. 7 was the heat pattern shown in FIG. 1, and the other test pieces were the heat pattern shown in FIG.

After completion of the low-temperature side quenching and cooling step, tempering was performed to 45HRC, and the presence or absence of tempering cracks, measurement of the amount of distortion, and Charpy impact value were evaluated.
The presence or absence of fire cracks was checked by a color check to see if there were cracks in the corners of the grooved part. For the measurement of the amount of distortion, the amount of deformation of the surface opposite to the grooved portion with respect to the reference surface was measured diagonally. Of the measured values, the amount that deviates most from the reference plane was divided by 300 mm and displayed as a percentage (%). The Charpy test piece was cut out from the center portion in parallel with the forging direction and subjected to a 2 mmU notch test.
Further, whether or not the crystal grain size was varied was confirmed by comprehensive judgment of 10 visual fields. The results of presence / absence of dispersion (presence / absence of mixed grain structure) are shown in Table 3 together with the results of the above-mentioned presence / absence of cracking, strain, crystal grain size, and Charpy impact value, and a representative micrograph is shown in FIG. 7) and FIG. 4 (Comparative Example No. 1).

The sample simulating the mold to which the method of the present invention was applied had no variation (mixed grain structure) and no burning cracking, and gave good results in strain, crystal grain size, and Charpy impact value. In addition, even when the micrographs of FIGS. 7 has a fine and uniform metal structure up to the center. In No. 1, crystal grains are rough and a bainite structure is developed.
Comparative Example No. In No. 1, since the heating rate from A1 to A3 was slow, the austenite grains grew slowly and the crystal grains were coarse, and the Charpy impact value was low and less than 20 J / cm ² . Comparative Example No. 8 and no. In No. 9, since quenching interruption was not carried out, there was no soaking process during cooling, cooling unevenness increased, and cracks occurred in the stress concentrated portion at the groove corner.

The mold to which the quenching method of the present invention is applied has fine crystal grains and high toughness, and can prevent large cracks when a large load is applied to the mold.
Further, since the quenching temperature can be increased, the hardness and the high temperature hardness are also high, which is effective in suppressing heat cracks and the like. Applicable to applications where high toughness and high temperature strength are essential. Further, practically, the reduction of the heat treatment distortion is effective in reducing the number of rework steps after the heat treatment.
Therefore, in a complicated shape such as a mold and a large steel material, it can be applied to a steel material for a purpose that requires refinement and uniformity of crystal grains.

It is a figure of the heat pattern which shows an example of the hardening method of this invention. It is a figure of the heat pattern which shows an example of the hardening method of this invention. It is a cross-sectional microscope picture of the sample which applied the hardening method of this invention. It is a cross-sectional microscope picture of a comparative example sample.

Explanation of symbols

1. Low temperature side quenching temperature increasing step 1A. 1. Quenching temperature rising preheating process 2. Quenching temperature raising step Holding step 4. 4. Quenching and cooling step 5. Interruption holding process Low-temperature quenching and cooling process

Claims (4)

In the mold quenching method, after the quenching and heating step in which the temperature range from the A1 transformation point to the A3 transformation point is heated at a heating rate of 100 ° C./H or higher, the temperature is maintained at a temperature range that does not exceed 1150 ° C. above the A3 transformation point. The holding step is performed, and then the quenching cooling step is performed at a cooling rate of 5 to 20 ° C./min in the temperature range from the A3 transformation point to 600 ° C., and 0.5 to 5 in the temperature range of 500 to 400 ° C. A die quenching method comprising a low temperature side quenching cooling step of cooling a temperature range of 400 to 200 ° C. at a cooling rate of 1 to 15 ° C./min after passing through a time interruption holding step. The method for quenching a mold according to claim 1, wherein the quenching temperature raising step of heating the temperature range from the A1 transformation point to the A3 transformation point is performed at a heating rate of 150 ° C / H or more. The heating in the low temperature side quenching temperature raising step performed before the quenching temperature raising step of heating the temperature range from the A1 transformation point to the A3 transformation point is performed at a heating rate of 150 ° C / H or less. 2. A method of quenching the mold according to 2. The quenching temperature rising preheating step for holding at least once is performed before the quenching temperature raising step for heating the temperature range from the A1 transformation point to the A3 transformation point. How to quench molds.